The interior part of my 3rd brake light was melted as well. When I was getting a replacement from the junk yard, same thing, melted. Seems to be a common problem. I put 3 of those small panels of LED's in there, and will never have that problem again
You can either do what I did, or just purchase an LED bulb with the correct socket to fit in there. I preferred the look of the panel, as it fills the brake light evenly, and not just a glowing bulb in the center.You can read all the details in this post in my build thread.
I used 3 of LED panels, end to end, and wired them all together. These are the panels I used
[This message has been edited by JohnWPB (edited 03-23-2015).]